I have the path to PHP set properly, and am able to run and debug through PhpStorm. If someone can help me understand what's going on here, I'd appreciate it. I have searched Google and only found one result, which was for Windows, not Mac. (https://devnet.jetbrains.com/thread/475384)

As you can see from your screenshot the "New Composer Project" screen does not have any option to select the PHP Interpreter that is required to actually launch composer (that's how PhpStorm works -- the same approach for all supported OS: Linux, Mac and Windows).

The workaround is to choose default PHP Interpreter in Default Settings (settings that would be used for any future new project created on this machine) -- this way you will have PHP Interpreter already pre-selected even for this kind of new project.

You need to use File | Default Settings to set such default PHP Interpreter there (well, that's on Windows/Linux -- on Mac it could be at some different location in menu). If you do not have any projects opened you still can access such Default Settings from Welcome screen.

[UPDATE]
Note: If you are still seeing same error after setting PHP Interpreter, then you'll need to restart PHP Storm and it will work like charm. (It worked for me)

In current versions (current 2016.1.x and upcoming 2016.2) you can specify what PHP Interpreter to use (and create new one, e.g. if you have none yet) at the bottom of this New Project screen -- it has Settings section which you can unfold -- just scroll the window down to see it.

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